The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.
The present invention relates to the field of permanent magnet structures and, more particularly, to the use of permanent magnet structures in electronic devices such as electromagnetic radiation sources.
One type of electromagnetic radiation source that uses permanent magnets is a free electron laser. A free electron laser is a device that passes an electron beam through a periodic magnetic field to produce so-called coherent light. Coherent light results when the strength and periodicity of the field is such that the electrons in different parts of the beam radiate coherently.
One type of structure that has been used to supply the field required to generate coherent light is a so-called wiggler. A wiggler is typically composed of pieces or segments of permanent magnets that are arranged along an axis so that their magnetic orientations are perpendicular to the axis and such that immediately adjacent permanent magnet segments have magnetic orientations that are opposite in direction to each other. Such a composition enables the wiggler to generate a periodically alternating magnetic field that is transverse to the axis of the wiggler. Accordingly, an electron passing along the axis of the wiggler is subject to a repetitive motion (i.e. the alternating magnetic fields). The repetitive motion of the electron, or acceleration of an electric charge, causes the electron to radiate with the frequency of the period of the alternating magnetic fields in the wiggler. By proper adjustment of the strength and period of the wiggler""s alternating fields, the electrons can be made to radiate such that they lase, and thus produce coherent light.
The light emitted from a free electron laser using such a wiggler, however, may contain many unwanted harmonics. The harmonics are due to the physical characteristics of the magnetic field that the wiggler produces. Specifically, the harmonics are due to the abrupt reversal of the magnetic field, from permanent magnet segment to permanent magnet segment, along the axis of the wiggler. The abrupt reversal from permanent magnet segment to permanent magnet segment causes the magnetic field to have a square wave distribution along the axis of the wiggler. Since such a square wave can be approximated by a sum of harmonics, the square wave distribution of the magnetic field along the axis of the wiggler causes the light emitted from the free electron laser to contain some harmonicity.
Free electron lasers that provide such harmonicity in the emitted light are not desirable in applications that require the free electron laser to output substantially monotonic light.
The present invention is a wiggler that, when used in a free electron laser, reduces the harmonicity of the light emitted from the free electron laser. In general, the reduced harmonicity is accomplished by making the magnetic field distribution of the wiggler to be sinusoidal, rather than square wave. A sinusoidal magnetic field distribution reduces the number of harmonics that must be used to approximate the magnetic field distribution of the wiggler, and thus reduces the harmonicity of the light produced by passing an electron beam through the wiggler. In particular embodiments, the wiggler is composed of pieces or segments of permanent magnets that are arranged along an axis such that their magnetic orientations are perpendicular to the axis, and such that the magnetic field distribution along the axis is sinusoidal.